As fully described in U.S. Pat. No. 5,530,702 to Palmer et al., a significant difficulty in establishing a multipoint communications system using transponders is how to prevent the transponders from attempting to communicate with the host at the same time. Such common transmissions usually cause a “collision” among the various transmissions and thereby corrupts the transmitted messages and renders them un-intelligible to the host system. In the '702 Patent, upon receipt of a “begin” signal transmitted by the host computer (through an interrogator), each transponder generates a random number and initiates an internal counter. When the count of the internal counter matches the random number, the transponder transmits a “request to transmit” signal to the host computer. Upon receipt of such “request to transmit” signal, the host computer transmits a first acknowledgment signal to all of the transponders. If the transponder which has transmitted the “request to transmit” signal receives this first acknowledgment signal within a predetermined period of time, it concludes that it has been given permission to transmit, and transmits an information-based signal to the host computer. If the transponder does not receive the first acknowledgment signal within the predetermined period of time, it generates a new random number and initiates the counter again. In this manner, a large body of transponders may communicate with a host computer. A problem with this approach, as with any approach utilizing random response delays, is that there must be a good estimate of the total transponder population. If there are fewer transponders than expected in the field, this approach will spend an unduly long time waiting for responses, while if the population is too great, this method will be contending with large numbers of collisions for long periods of time.
Other systems for establishing communications between a host computer and a plurality of transponders are disclosed in U.S. Pat. Nos. 5,519,381, 5,550,547, 5,986,570 and 6,034,603. However, as discussed briefly below, each of these systems possesses certain drawbacks in operation that prevent optimum performance. The '381 Patent discloses an identification system having an interrogator and a plurality of transponders, but requires that the interrogator sequentially transmit two different signals to the transponders using two different antennas, to ensure that all transponders are identified, necessitating additional expense and complexity in the design and construction of the interrogator. The '547 Patent discloses a “tree-splitting” algorithm for determining which transponder should transmit wherein the interrogator first transmits a read command and then transmits a failure notice upon the detection of a collision. Approximately half of the transponders will not transmit thereafter, based upon certain internal operations keyed to an internally-generated random number. This operation continues until no failures (collisions) are detected, and then the transponders begin to communicate. This filtering process, during which no data is recovered by the interrogator, is time intensive when communication is with a large body of transponders. It is also time intensive because it is limited to only a single transponder response per one or more interrogator commands. Similarly, the '570 Patent discloses a system wherein the interrogator signals the transponders when a collision is detected, and the transponders cease communicating for a period of time established by a random number generated within the transponder. As such, the system of the '570 Patent also provides satisfactory results, but also is time intensive when random wait periods exceed the minimal statistical requirements of the transponder population, and therefore is not optimal. Finally, the '603 Patent discloses a system in which each transponder includes circuitry which is able to detect transmissions by other transponders, and each transponder only transmits when it detects no other transmissions. If a transmission by another transponder is detected, each transponder waits a predetermined time before listening for competing transmissions again. Although the '603 Patent provides adequate operation, it requires transponders having additional complexity (and therefore additional cost) for the circuitry required to receive transmissions from the other transponders and is also time intensive when faced with a large body of transponders, and thereby presents certain drawbacks.
Another drawback of conventional RFID systems relates to the spatial range between the interrogator and each transponder. Conventional RFID systems have been designed for a variety of categories of operation, all of which must be approved or licensed by regulatory agencies, e.g., the U.S. Federal Communications Commission (FCC) for systems intended for use in the United States. The FCC has approved conventional RFID systems, without requiring an operator's license, at various frequency bands and with maximum RF power levels specified for each frequency band. In approving such systems, the FCC has placed strict limitations on the carrier modulation and on the mode of signal returned from the RFID transponder. For example, most un-licensed systems may not include any form of RF amplification within the transponder of signals returned by the transponder to the interrogator.
One class of conventional RFID systems, disclosed in U.S. Pat. No. 5,053,774, has focused on reading a single transponder at close range. These systems are commonly known as proximity systems and have been designed to operate at carrier frequencies below 30 MHZ. In systems of this type, the transponder can receive its electrical power from the RF signal of the interrogator and no battery is required in the transponder. There are currently several such systems on the market, including the TIRIS™ system from Texas Instruments, Inc. These systems currently are marketed for security cards, money cards, animal identification, etc. The drawbacks of this type of system are that only a single transponder may be addressed at one time and that the spatial distance between the interrogator and the transponder is very small.
Another class of conventional RFID systems, disclosed in U.S. Pat. No. 5,030,807, has focused on reading a single transponder on moving objects, e.g., for vehicle identification and automatic toll collection. These systems operate at ranges up to 6 meters, but are not designed to read more than one transponder.
A further class of conventional RFID systems, disclosed in U.S. Pat. Nos. 5,640,683, 5,649,295 and 5,649,296 focused on communicating with a plurality of transponders by frequency shift keying (FSK) the backscatter (i.e., re-radiated) signal from a particular transponder. A precision (e.g., crystal) oscillator is located on the transponder, and the backscatter antenna of the transponder is voltage controlled by a signal at a frequency derived from that precision oscillator. The resulting reflected signal contains a subcarrier that is offset in frequency from the signal originating from the interrogator. The offset frequency subcarrier signal is itself modulated according to the data being transmitted by the transponder. Although this type of system increases the spatial range between the interrogator and the transponders and allows the background hum due to residual reflection by non-communicating transponders of the signal originating from the interrogator to be filtered away, the data communication rate is quite low (e.g., 1 kbps) because it is necessarily at a low frequency in comparison with the subcarrier.
Yet another class of conventional RFID systems, disclosed in U.S. Pat. No. 5,828,693, has focused on batch reading of transponders using frequency hopping as a spread-spectrum communication means. These systems operate at UHF frequencies (915 MHZ) or microwave frequencies (2.45 GHz or 5.8 GHz) but require complicated frequency hopping circuitry.
Yet a further class of conventional RFID systems, disclosed in U.S. Pat. Nos. 5,539,775, 5,825,806 and 5,974,078, has focused on batch reading of very small transponders at ranges under 2 meters. These systems operate by amplitude modulating and phase modulating a microwave carrier (2.45 GHz or 5.8 GHz), in order to minimize the size of the antennas. However, because each antenna (there could be more than one) is smaller than resonant antennas at lower frequencies, the power received by the antenna is less and the read/write ranges are less than for similar systems operating at lower frequencies. In order to compensate for the weaker signal strength, the RF signal detection and demodulation of the RFID transponder of these systems is considerably more complex than that of the present invention.
The use of a direct sequence spread spectrum (DSSS) signal transmitted from the interrogator to the transponder in RFID systems has been complicated by the need for complex and power intensive demodulation strategies in the transponder. The system disclosed in U.S. Pat. No. 5,974,278 first creates an amplitude modulated signal with the modulation being a direct sequence waveform. The transponder demodulates the received signal in two steps. First, the amplitude modulated waveform is detected by an AM Detector, and the presence of detected signal energy is used to turn on a data correlator, which then processes the baseband direct sequence signal. The system of the '278 Patent is deficient in that it is susceptible to jamming from other on channel carriers, including frequency hopped spread spectrum (FHSS) signals, has long acquisition times (see Column 5, line 60: “. . . several hundred data bit periods.”), and requires a high speed onboard clock to clock the onboard PN generators. The system described in the '278 Patent also uses carrier regeneration to return signals to the interrogator and is thus not a reflective transponder.
An alternative to the DSSS system of the '278 Patent is to use the DSSS system and method disclosed in U.S. Pat. No. 5,559,828, which sends a pseudo-noise (PN) code reference together with PN coded data on the in-phase and quadrature phases of the same carrier frequency. With this method, sometimes referred to as a Quadrature Fast Acquisition Spread Spectrum Technology (QFAST®) system, there is no need for the transponder to have an on-board code generator to recorrelate the coded signal to the original data bandwidth. These QFAST® systems demodulate with a simple delay-and-multiply strategy, which may be implemented with passive analog delay components. However, until now the DSSS system and method of the '828 Patent has not been adapted for use in an RFID system.
One problem facing conventional RFID systems is interrogator receiver desensitization caused by the interrogator transmitter itself. Methods to mitigate the deleterious effects of this local strong signal have been described in the field of RADAR and involve a technique called range gating. The first range gating system was implemented on pulsed RADAR. The RADAR illuminator system clamped off the receiver, and sent out a very short burst of high power RF. After the transmitter was shut off, the receiver input was reactivated, and used to listen for the returned pulse. A refinement on this technique opened the receiver for a specific period, beginning after a controllable delay. Then anything received could only have come from an object a known distance away, inside the “range gate”. A problem with the pulsed RADAR system was that the power in the transmitted pulse had to be quite high in order for the received signal to have a reasonable signal to noise ratio. The combination of wide receiver bandwidth to accommodate the short pulse, and the brief integration time for a single pulse, ultimately limited the range of early pulsed RADAR systems.
A second range gating system utilized a continuous RF signal whose center frequency was changed continuously. A returned signal even from a stationary object, would have a different frequency than the current transmitted frequency or receiver Local Oscillator. This frequency offset resulted in a synthetic Doppler component in the demodulator output, with the range to the object being “encoded” in the amount of frequency offset. This type of system is referred to as a “chirp” system. While this system overcame the need for high peak power, and provided long integration times, it was easily jammed by the presence of other emitters in the band. Placing a secondary phase modulation on the transmitted signal, which could be checked, reduced the jamming problem. Subsequently, the use of a special digital pattern, called a Barker Sequence, improved the ranging resolution on the signal through its unique auto-correlation properties. Later, with the advent of high speed digital circuitry, it became possible to generate long “pseudo-noise” codes which were deterministic, but which would not repeat on a timescale similar to the expected returning signal. Such systems provided range gating through digital direct sequence modulation. The receiver in such a system utilized both the knowledge of the transmitter center frequency and the displacement of the code phase to provide range gating and Doppler rate determination.
A further problem facing conventional RFID systems is that, in order to read RFID transponders that may be distributed over an area that is considerably larger than the spatial range of a single interrogator antenna assembly, either an interrogator must roam the area or fixed-position interrogators must be positioned in an array located within the area. The latter configuration is preferred in most automated applications. However, if adjacent interrogators in such an array are used simultaneously to read data from transponders in the area, their signals might possibly interfere with each other. Therefore, it is usually necessary to operate the multiple interrogators one at time in a sequential manner. In some applications, a typical floor space may require 500 or more interrogators. If only one interrogator were to be operated at a time, and if each interrogator required several minutes to complete the reading from, or writing to, the RFID transponders within its range, then the entire operation could take several hours to complete. In many situations, it is important to complete this operation as fast as possible so that other tasks can be permitted to resume.
Various objects, advantages and features of the present invention will become readily apparent from the ensuing detailed description and the novel features will be particularly pointed out in the appended claims.